We all know that movies are inaccurate — to time, to science, to logic, to history.

We also know that sometimes it matters, and sometimes it doesn't, but we disagree there. The line between what matters and what doesn't is not only different for everybody, but it also varies within every person. For example, if you're an expert on something, small errors will leap out at you that you might otherwise not notice, or ignore.

In October, the astrophysicist Neil deGrasse Tyson made some Twitter comments about the movie "Gravity" that were repeated everywhere. Some of what Tyson called "Mysteries of Gravity" were the kind of things that only an astrophysicist would care about: "Nearly all satellites orbit Earth west to east, yet all satellite debris portrayed east to west."

Other mysteries were just funny: "Why (Sandra) Bullock's hair, in otherwise convincing zero G(ravity) scenes, did not float freely on her head." Obviously, that mystery isn't a real mystery to anyone. A Hollywood leading lady cannot have a bad hair day, not in the Wild West, not in a concentration camp, not even in outer space.

But one of Tyson's tweets was especially arresting because it went to the heart of the movie, touching on the most haunting and dramatic moment of the entire story. (In fact, if you haven't seen the movie yet, skip the next two paragraphs, because they contain a key spoiler.) Tyson writes, "When (George) Clooney releases Bullock's tether, he drifts away. In zero-G, a single tug brings them together."

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Of course. We should have known that. What's more, Bullock's character should have known that. The only explanation for her not knowing is that she wanted to be the star of the movie, and she couldn't do that with Clooney floating around getting all the best lines. Obviously, given the trajectory of "Gravity," we know why Clooney had to go, but it does cast that painful moment in a strange other light. Scientifically (if not dramaturgically), that did not have to happen.

Sometimes a little knowledge can get in the way of an experience. As someone who grew up in New York, I sometimes have to remind myself that not everyone in an audience is driven up the wall by an inaccurate New York accent. On the other hand, sometimes it does matter. If it's one character sounding strange, fine. But if you're making a movie about an entire subculture — as in "Moonstruck" or "World Trade Center" — get it right.

Last fall, Rick Spilman, in the Old Salt Blog, criticized "All Is Lost" for Robert Redford's not having an EPIRB (emergency position indicating radio beacon) on board. Apparently, there's no way anybody would undertake that journey without one. Of course, if you know nothing about sailing, it makes it easier to ignore such details.

Alfred Hitchcock had a pet name for critics. He used to call them "the Plausibles," because they often criticized his films for being "implausible." You still see that word pop up in criticism today. Movies are not about the plausible. We get enough plausible in real life.

Yet, even if you accept this, there are certain things that will stick out and remind us that we're watching a movie. In Hitchcock's own "Foreign Correspondent" (1940), for example, you can see the set lights during a plane crash sequence. That was just a (barely noticeable) mistake, like the wristwatch on an extra in "Spartacus." Or a bust of Woodrow Wilson and an electrical outlet in the foreground in "Lincoln."

And then there are anachronisms, such as the kilts in "Braveheart." Scotsmen didn't wear kilts until hundreds of years later. Who knew? Or the Granny Smith apples and the sweet bananas in "Pirates of the Caribbean": They didn't exist in the 18th century.

Those are honest errors. Other errors are intentional, but understandable. Distortions of distant history for the sake of drama can be excused. Less forgivable are distortions of more recent history done with political intent. The classic example of this is William Dieterle's "Tennessee Johnson" (1942), which purports to tell the life of Andrew Johnson, the 17th president of the United States. The movie distorts the history of Reconstruction, makes the Radical Republicans into fiends. It leaves out the part about Johnson being a boor and a racist who set back civil rights for many decades.

On the other end of the spectrum, last year's "The Butler" took the true story of an actual African American butler in the White House and changed everything about him, including his name. They gave him two sons, instead of one. They had one son get killed in Vietnam. That didn't happen. And they gave another son an unlikely career as a Freedom Rider turned Black Panther turned congressional candidate turned antiapartheid activist. None of that happened, either. They even had the butler scowling through a state dinner, to which he'd been invited by Nancy Reagan, even though the real-life butler had a good time at the dinner.

Fiction is great, not in spite of itself, but because it's fictional, because it's untethered. The only caution I'd add is that sometimes accuracy can imply an opportunity worth considering: What would have happened had Bullock given Clooney a little tug? What might have happened if the butler were allowed to really like Nancy Reagan?